The evolution of REE and Pb isotopes during soil development, Wind River Mts., WY
Abstract
The isotopic composition of Pb, Nd-Sm and the distribution of major and trace metals and REE in six soil profiles varying in age from ∼0.4 to ge300 kyr were investigated in order to determine their usefulness for relative soil dating. These soil samples were collected from a series of glacial moraines developed on the slops of the Wind River Mountains, Wyoming. The elemental composition of the acid-leach fractions of these soils was used to define distinct soil horizon (A, B and C) for each profile. The behavior of the various elements and isotopic systems was then studied in each of the horizons. In each of the soil horizons (A, B and C) the Pb isotope ratios (206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb) decrease with soil age, with some exceptions. However, the amount of radiogenic lead (Pb^*, 208Pb^*, and 206Pb^*) in the acid-leach fraction of the deepest sample decreases systematically with soil age. The initial values for the calculated ^*Pb in the acid-leach are the whole soil Pb composition values. The Pb^* in the soil labile pool of the C-horizon samples, is probably the best criteria for applying Pb isotopes to relative soil dating. Based on the Pb isotope ratios, it is possible to assess whether weathering of U or Th-rich phases are the major sources of radiogenic Pb in the soil labile pool. On a 208Pb/204Pb versus 206Pb/204Pb diagram the two ratios behave differently with soil age. The three youngest soils (0.4-12 kyr) have high values of 206Pb/204Pb and 208Pb/204Pb ratios and are scattered along the 208Pb-206Pb diagram line, reflecting a significant contribution of Pb from weathering of less resistance accessory phases (U and Th-rich phases). Next, the 208Pb/204Pb ratio in soils of 22 and 138 kyr old decreases sharply while the 206Pb/204Pb ratio remain almost constant. Finally, a reverse behavior is observed in the oldest soil (>300 kyr) where the 206Pb/204Pb ratio decreases sharply while the 208Pb/204Pb ratio remains nearly constant. The chondrite-normalized REE patterns of soil acid leach and digestions of the deepest samples reflect the till's composition. The LREE's are depleted in the soil profile due to early dissolution of LREE enriched phases such as allanite. The 143Nd/144Nd ratio in three whole soil B-horizon samples decreases with soil age probably not reflecting changes in soil properties but because of initial differences in the mineral assemblage of the soil. There has been a growing interest in recent years in utilizing stable heavy isotopes such as Sr, Pb and Nd for the study of weathering. Based on the criteria developed in this study for Pb and Sm-Nd isotopic compositions and REE pattern it should be possible to assess the applicability of other isotope systems in studying mineral and rock weathering.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....8188H